Supernates containing optimal concentrations of low molecular weight inhibitor of lymphoid cell division were obtained by culturing spleen cells from young Brown Norway or Lewis rats under basal conditions at appropriately high cell densities. Spleen cells from other mammalian species (humans, mice, guinea pigs, rabbits and fetal calves) all released significant but somewhat lower inhibitory activities per unit spleen weight. Two malignant lymphoid cell lines, L1210 murine leukemia and MOPC-31-C plasmacytoma, were tested as potential sources for production, but released no detectable low molecular weight inhibitory activity. The rat splenic inhibitor produced a reversible, species-independent decrease in the DNA synthesis and mitosis of rapidly dividing normal thymic (T) and bursal (B) lymphocytes, without markedly altering their protein synthesis, or cell viability. The inhibition of DNA synthesis in human peripheral blood and splenic lymphocytes from patients with acute lymphocytic leukemia, undifferentiated acute myelogenous leukemia and reticular lymphoma was generally greater than or equivalent to that observed using normal lymphoid targets. The inhibitor was partially purified by Diaflo filtration, molecular filtration on Sephadex G-10 and ion exchange chromatography. The active fraction contained a heat-stable, anionic moiety in the molecular weight range of 300 to 800 daltons. BIBLIOGRAPHIC REFERENCES: Ranney, D.F., and Pincus, J.H. Suppression of Stimulating Cell Activity by Microtubule-disrupting Alkaloids. J. Supramolecular Structure (in press, Vol. 5, 1977). Lopatin, D.E., and Ranney, D.F. Maintenance of the Resting State and Potential Regulators of the Proliferative Phase. In Regulatory Mechanisms in Lymphocyte Activation. (D.O. Lucas, Ed.), Academic Press, New York (in press, 1977).